BERKELEY, CA — A team led by Michael Crommie, a staff
scientist in Lawrence Berkeley National Laboratory's Materials Sciences
Division and a professor of physics at the University of California at
Berkeley, has used a scanning tunneling microscope (STM) to attach individual
potassium atoms to isolated carbon-60 molecules.

By using the STM's tip to move a C60 molecule over
potassium atoms one at a time, the buckyball can reliably be made
to acquire up to four potassium-atom dopants.

By adding potassium atoms to familiar soccer-ball-shaped "buckyballs,"
Crommie and his coworkers can increase the electric charge on each C60
molecule; individual potassium atoms are either attached or removed from
a C60 molecule using the tip of an STM. The method demonstrates that the
electronic properties of an individual molecular structure can be reversibly
tuned with atomic precision. The researchers report their work in the
12 March 2004 issue of the journal Science.

"Doping materials is a fundamental component of the entire modern
electronics industry," Crommie says, referring to the process of
adding impurities like phosphorus or boron to semiconductors like silicon
to give the doped material an excess of negatively charged electrons or
positively charged holes.

Junctions between n-type and p-type semiconductors are at the heart of
the diodes, transistors, integrated circuits, computer chips, and other
devices that make possible personal computers, cell phones, CD and DVD
players, solar cells, and hundreds of other electronic gadgets. Doping
bulk materials is well understood, says Crommie, "but we need to
take these techniques and scale them all the way down to the single-molecule
level."

At this level, one or two extra electrons' worth of charge can affect
the performance of critical electronic components. Building a molecular
p-type/n-type junction might require electron doping of one molecule and
hole doping of another. "With this work we've shown how to control
the electron doping with absolute precision," Crommie says.

Crystals and monolayers of buckyballs and other fullerenes have long
been doped by introducing metal atoms like potassium or rubidium. Crommie
and his colleagues extended the process to the atomic level by depositing
widely separated C60 molecules and potassium atoms on the surface of a
silver crystal polished to virtually perfect flatness. The samples were
prepared in ultrahigh vacuum and cooled in an STM to just seven degrees
Kelvin above absolute zero.

In an STM a voltage bias between a fine probe, only a few atoms wide
at its tip, and the surface of the sample being investigated causes an
electric current to tunnel between them. The strength of the current yields
information about the sample's microscopic shape and electronic structure.
And by bringing the tip close enough to attract individual atoms or molecules,
they can be moved at will.

As a buckyball acquires potassium-atom dopants the
energy state of its molecular orbitals shifts, causing the doped molecule
to "light up" in the STM image.

After depositing C60 molecules and potassium atoms on the silver surface,
Crommie's group mapped their positions, then used the STM's tip to move
the buckyballs over the potassium atoms, picking them up one at a time
 like a molecular Pac-Man. In this way a buckyball could reliably
be made to pick up from one to four potassium atoms. By then moving the
buckyball over an impurity in the silver surface (most likely an oxygen
atom), the potassium atoms could be "pulled off" one at a time.

The shape of an individual C60 molecule did not change significantly
when potassium atoms were added: a buckyball with four potassium atoms
was about nine percent wider and three percent shorter than an unadorned
buckyball.

Electronic changes were more marked. The added charge donated by the
potassium atoms caused the molecular orbital states of the C60 molecule
to fill with electrons in a way analogous to the way the conduction band
of a semiconductor fills with electrons when it is n-doped. Unlike the
potassium-doping of C60 in extended monolayers and bulk crystals, however,
where potassium atoms contribute one electron each, here each potassium
atom contributed only about 0.6 of an electron's charge to the individual
buckyball.

These results suggest that the potassium atoms collect at the interface
where the C60 molecule meets the silver surface, partially hiding from
the STM's probe and sharing part of their charge with the silver substrate.

Until now, controlling the electronic properties of molecular structures
in the developing field of nanotechnology (a nanometer is one billionth
of a meter) usually involved having chemists synthesize new starting materials
in a test-tube or "gating" the structure  i.e., influencing
the electronic environment with nearby electrodes. The new work offers
a third way, with the major advantage of flexibility.

"If you want molecular structures to jump up and dance, the name
of the game is control," says Crommie. "Tunability is the key
to tailoring the electronic properties of individual molecules. We have
demonstrated that we can do this in situ in a controllable, reversible
way."

"Controlled Atomic Doping of a Single C60 Molecule," by Ryan
Yamachika, Michael Grobis, Andre Wachowiak, and Michael F. Crommie, appears
in the 12 March 2004 issue of Science.

The Berkeley Lab is a U.S. Department of Energy national laboratory located
in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California. Visit our website at http://www.lbl.gov.